The goal of this application is to utilize opioid receptor knockout mice to answer several important questions concerning opioid receptor pharmacology. The initial series of experiments will explore and characterize the binding site responsible for the analgesic action of delta opioid ligands in DOR-1 KO mice. Concurrent studies using an inducible DOR-1 KO mouse will identify whether the effects if delta ligands in DOR-1 KO mice are due to compensatory mechanisms induced by ablation of the DOR-1 gene or unmasking of a hereto unknown delta opioid receptor system. These studies will clarify the identity of a receptor system involved in analgesia, which could then be exploited for therapeutic benefit. We will also determine whether the DOR-1 gene product can account for both pharmacologically defined delta opioid receptor subtypes by forming homodimers with itself or heterodimers with mu opioid receptor. Finally, as suggested by preliminary data we will examine whether the formation of morphine tolerance is correlated with a decrease in mu/ delta opioid receptor heterodimers. Confirmation and localization of in vivo mu/delta opioid heterodimers and clarification of a possible role for mu/delta heterodimers in development of morphine tolerance will help uncover treatments by which the analgesic action of morphine tolerance will help uncover treatments by which the analgesic action of morphine are not lost, but morphine dependence is lost.